The process of superplastic forming and diffusion bonding of metals is well known in the art. A summary of such a process and a system for accomplishing such a process is disclosed in U.S. Pat. No. 4,304,350 to Paez, et al., issued Dec. 8, 1981, to the present assignee. The present invention deals with an improvement of an inlet assembly, as will be discussed hereinafter.
Briefly, the process of superplastic forming and diffusion bonding has the advantages of forming complex shapes and permitting deep drawing of metallic parts. At the temperatures involved the deformation stresses are relatively low, which permits the forming of complex parts under pressures which keep tool deformation and wear at a minimum.
Diffusion bonding which is used in conjunction with superplastic forming refers to the metallurgical bonding or joining of metal by the steps of applying heat and pressure for periods of time sufficient to cause comingling of the metal atoms at the places where the surfaces are joined.
The superplastic forming and diffusion bonding process is particularly useful in the aerospace industry in the formation of wings, frames, and the like. The process is particularly useful in forming strengthening members used in aerospace structures referred to as frames and stringers.
Generally speaking, the process begins with cleaning the metal sheets to be used in the sandwich. The sheets are then selectively coated with a material commonly referred to as "stop-off" which typically is boronnitride, yttria, or some rare earth or other very inert material. The purpose of the "stop-off" is to prevent diffusion bonding of the sandwich-like material where the stop-off material has been applied. The areas coated with "stop-off" can be formed into a desired shape by the application of gas pressure applied to the inside of the metallic sandwich. Prior art systems have not been reliable for commercial production. In some prior art practices, an inlet tube was inserted between the sandwich layers to be used to evacuate the space between the sandwich layers and to thereafter supply gas under pressure to the spaces between the sandwich to form the sandwich into the desired honeycomb shape by superplastic forming.
In prior practices, after inserting the tube between titanium sandwich layers, the sandwich construction sometimes was welded around its periphery to form a unitary structure or was bonded during the superplastic forming, diffusion bonding cycle to form an assembly sometimes called a bag. Typically, this was accomplished by seam welding or under heat and pressure by diffusion bonding. In many operations there were more than one such inlet tubes.
One problem that has arisen as a result of these prior art practices where the inlet tube extended into the sandwich was that, while welding the peripheries of the sandwiched sheets, one or more of the inlet tubes would be pinched closed and rendered inoperable. Where this occurred, the subsequent operations were severely adversely affected, sometimes resulting in complete failure to form the desired assembly.
In another approach a tube was welded to the bag. The weld is susceptible to cracks that could break the seal which should exist between the inlet tube and the sandwich construction which also would adversely affect the superplastic forming operation.
In the previously identified U.S. Pat. No. 4,304,350, an improved system and process relating to titanium sheets is disclosed. That invention includes upper and lower die members which receive the sandwich assembly. The die members include passageways adapted to direct gases to and from the sandwich assembly. One of the die members has a recess in which is positioned a hollow metal pin preferably made of stainless steel. One end of the hollow pin is in sealing relationship within an opening in the bottom layer or sheet of the sandwich. The other end of the hollow pin has a knife-like edge and is in sealing relationship with a washer-like member within the recess. A feature of the invention is that the stainless steel tube is directed to the sandwich in a direction perpendicular to the sandwich rather than parallel to the sandwich.
In operation, the hollow pin is press fitted in the hole in the bottom sheet of the sandwich. The sheets are then cleaned and stop-off, preferably boron-nitride, selectively applied to the sheets to determine the shape of the structural article and the areas of diffusion bonding of the sheets.
The titanium sandwich sheets are then seam welded around the periphery to form an assembly. The assembly is placed in a cavity formed by dies and a seal effected between the hollow stainless steel pin and the die in which it is located. The dies are brought together to apply a squeezing pressure to that portion of the assembly to be diffusion bonded. The assembly is heated to about 1650.degree. F. and the sandwich is subjected to an inert gas, preferably argon, to prevent contamination of the sandwich surfaces. Thereafter, the assembly is heated for a prolonged period at about 1650.degree. F. while inert gas under pressure is directed to the interior of the sandwich assembly to form a structural assembly.